Expandable var1 gene of yeast mitochondrial DNA: in-frame insertions can explain the strain-specific protein size polymorphisms.

The var1 locus of yeast mitochondrial DNA encodes a protein of the small mitochondrial ribosome subunit, denoted var1 protein. The size of var1 protein was previously shown to exhibit a strain-dependent polymorphism, determined by various combinations of at least three genetic elements. We report here that the var1 gene is itself polymorphic and that the six forms of this gene examined here differ by various combinations of three in-frame insertions into the coding region of the smallest allele. These insertions, which appear to be the molecular basis for the genetic elements, could increase the size of var1 protein by 8, 10, 16, 24, or 26 amino acid residues, accounting for the observed protein polymorphisms. Furthermore, we have characterized three additional sources of sequence variation located outside of the coding region but within major transcripts of the var1 gene.